DE10051049A1 - Semiconductor substrate made from silicon carbide has a p-doped layer, a silicon dioxide layer grown by depositing from the gas phase onto a p-doped silicon carbide layer, and several contact structures applied using photolithography - Google Patents

Abstract

Semiconductor substrate made from silicon carbide has a p-doped layer; a silicon dioxide layer grown by depositing from the gas phase onto a p-doped silicon carbide layer; and several contact structures applied using photolithography. The contact structures are formed by a layer of aluminum-nickel containing 40-60 at.% aluminum, the layer being deposited in contact openings. The contact structures are heat treated to form an ohmic contact resistance between a p-doped silicon carbide layer and an aluminum nickel layer at 800-1200 deg C. An Independent claim is also included for a process for the production of a semiconductor substrate. Preferred Features: The aluminum-nickel layer is deposited on the substrate by sputtering on an aluminum-nickel target in the gas phase or by diffusion tempering an aluminum-nickel multi-layer.

Description

The invention relates to a product with the features of the independent claim.

So far, aluminum has been used for p-doping SiC substrates. It was therefore obvious Aluminum can also be used as a contact material for p-SiC. Appropriate experiments with Al contacts on p-doped SiC substrates showed both in the initial state Deposition from the gas phase and also after the contact composite has been tempered for one hour no resistive behavior at 550 ° C, d. H. a linear current-voltage relationship the contact resistance. When the temperature rises, the aluminum contact melts as expected. Ohmic contacts can therefore be made with aluminum on p- not reach doped SiC.

In the past, therefore, contact materials were used that were suitable for n-doped SiC Substrates are suitable, examined for their suitability for p-doped SiC substrates. tries with nickel, titanium or titanium silicide, however, always led to p-doped SiC substrates Contacts with non-linear current-voltage characteristics, so that these metals as ohm Contacting for p-doped SiC substrates must be excluded.

Exponential current-voltage characteristics of contact metallizations always lead to a disproportionate increase in the current density under the contacts. Hereby comes In semiconductor devices, there is an undesirable loss of power at the contacts. Also there is a risk of severe local overheating under the contact, making it bad at best, the contact can be destroyed.

Starting from the previously described state of the art, the inventive method emerges Task an ohmic contact metallization for p-doped SiC substrates and a Kon Specify clocking procedures to achieve these ohmic contacts.  

According to the invention, this object is achieved by the features of the independent An entitlement. Further advantageous embodiments are in the subclaims and in Description included.

The solution is achieved through an aluminum-nickel connection with an aluminum content in the range from 40 at% to 60 at% (atomic percent), particularly preferably with a proportion of 50 at% aluminum and 50 at% nickel. This AlNi contact metallization is derived from the Gas phase deposited on the p-doped SiC substrate and the composite of Konatktmetalli Sation and p-doped SiC substrate then heat treated. The heat treatment The composite consists of a tempering in the temperature range from 800 ° C to 1200 ° C, preferably in the range from 1000 ° C to 1200 ° C and particularly preferably at 1200 ° C and serves to establish an ohmic contact between the AlNi contact metallization and the p-doped SiC substrate.

The AlNi metallization is achieved by depositing the metals from the gas phase on the p- generated doped SiC substrate. Here, in one embodiment of the invention both components of the metallization by sputtering from an AlNi target onto the p- doped SiC layer deposited. The AlNi target consists of an AlNi alloy with an aluminum content in the range of 40 at% to 60 at%.

In another embodiment of the invention, the two metals aluminum and Nickel deposited separately on the p-doped SiC substrate. Then it will be first Mulilayer layers formed with alternating layers of aluminum and nickel. The The multilayer layer is then subjected to diffusion annealing at 550 ° C. for 2 hours subjected. In diffusion tempering, the multilayer layer creates a uniform aluminum Ni contact metallization in the desired composition with an aluminum An  part in the range from 40 at% to 60 at%. The diff Mier tempering from the aforementioned embodiment of the invention.

The main advantages of the invention are as follows:

With the aluminum-nickel contact metallization according to the invention of p-doped silicon Ohmic contacting of semiconductor components is reliably achieved on substrates the basis of SiC substrates. The proportion of rejects in contacting SiC Semiconductor components are reduced by the invention. It becomes reliably ohmic Contacts achieved compared to contacts with exponential current Voltage characteristics have advantages; namely a lower drop in performance at the Kon clocking and less heat generation. An undesirable increase in electricity density under the contact is also avoided.

The connection can also be established on the aluminum-nickel contact metallization conventional construction and connection techniques to be continued. So you can then Eating layers further bond metallizations or passivations or protective metallization be built up.

Embodiments of the invention are illustrated below with reference to drawings provides and explained in more detail. Show it:

Fig. 1 shows a schematic cross section of a SiC substrate with aluminum-nickel contact metallization:

• A) contact structures prepared using shadow masks,
• B) contact structures produced by photolithography,

Fig. 2 is a formal distribution of the Al-Ni-layers and layers for the game of Ausführungsbei Diffusionstemperns of Mulilayer structures,

Fig. 3 shows examples of measured sheet resistances of the Al-Ni contact metalization in depen dependence of the Al content on SiC or SiO ₂ substrates by treatment of different heat,

Fig. 4 current-voltage characteristics between contacts AlNi with different Al content after one hour Diffusionstemperung at 550 ° C,

Fig. 5 shows the dependence of the specific contact resistance of AlNi- contact metallizations on the Al content, the contacts were fusionsgetempert 60 min at 550 ° C and dif 2 formiergetempert min at 1200 ° C,

Fig. 6 shows a schematic cross section through a contact on p-SiC with AlNi 50:50 contact metallization, barrier layer, bond layer, SiO ₂ passivation and protective metallization.

Fig. 1 shows two schematic structures of a contact metallization on a semiconductor substrate. A setup using shadow masks and a setup using photolithography and SiO ₂ passivation.

The decisive electrical parameter of an ohmic contact is its specific one Contact resistance. In addition, the contact metallization should also have a low specificity have layer resistance.

The use of shadow masks is technologically simpler and does not require SiO ₂ passivation. However, the shadow masks were only suitable for producing relatively coarse geometries with a structure width of at least 200 µm and, due to the process, always produced slightly blurred edges.

In contrast, the SiO ₂ layer could be structured with high lateral accuracy during the photolithographic sample preparation and thus contacts with widths down to 1 µm and sharp contours could be produced.

SiC wafers from Cree Research Inc., which consisted of n-doped SiC substrates on which a 1 µm thick and p-doped aluminum SiC layer was grown epitaxially, were used as the starting material for the substrates for both production processes was. The net doping of the epitaxial layer (difference between acceptor and donor concentration) had been determined by Cree by CV measurements on an Hg prober and was between 1.0 and 2.0.10 ¹⁹ cm ^-3 .

The wafers were cleaned in H ₂ O ₂ / H ₂ SO ₄ and in concentrated hydrofluoric acid. After cleaning, a 1.5 µm thick SiO ₂ passivation is deposited on the substrates by CVD (chemical vapor depositon) at atmospheric pressure. In a second lithography step, contact openings were etched free by RIE in this passivation. The metallization of the substrates with AlNi multilayers is carried out in a vapor deposition system by electron beam evaporation of aluminum and nickel, the integral aluminum content of the aluminum-nickel layer being varied between 0% and 100%. In a preferred embodiment of the invention, the metallization of the substrates is carried out by electron beam evaporation of an aluminum-nickel alloy with an aluminum content between 40 at% to 60 at%.

Fig. 2 shows a particular embodiment to achieve an aluminum contact metalization, namely a formal layer structure for producing an aluminum-nickel contact metallization according to the invention from a multilayer layer. The multilayer layer is diffusion annealed to produce a homogeneous aluminum-nickel contact metallization. The diffusion annealing process can be imagined as follows:

With a purely formal division of the nickel layers of an Al-Ni multilayer onto each because the aluminum layers directly adjacent to the nickel layer would be the upper aluminum the entire upper nickel layer plus half of the middle nickel layer orderly. The middle nickel layer would account for half of the middle and the un only nickel and the lower aluminum layer would come only half of the lower Nickel layer. The Al-Ni multilayer is therefore rich in nickel in the upper part and in the lower part aluminum rich.

Nickel is the more mobile species in the AlNi system. It is therefore understandable that the upper aluminum layer due to the 1.5 assigned nickel layers during tempering can be "supplied" with nickel faster than the middle aluminum layer with one assigned nickel layer or the lower aluminum layer with only half assigned neten nickel layer.

For the systematic investigation of AlNi multilayer metallizations, the Ge total stoichiometry of the multilayer by adjusting the layer thickness of the individual alumini layers or nickel layers vary from pure aluminum in 10% steps to pure nickel, here and in the following to be understood as atomic percentages for all percentages are.

The multilayer consisted of three identical Al-Ni double layers with an aluminum layer at the interface to the substrate. The densities and relative atomic masses of aluminum (ρ = 2.702 kg / dm ³ , A _r = 26.98154) and nickel (ρ = 8.8 kg / dm ³ , A _r = 58.69) were used to determine the layer thicknesses of the individual Aluminum and nickel layers for the different integral stoichiometries calculated so that there was always a total layer thickness of 400 nm. The results are summarized in Table 1 together with the names of the metallizations.

Table 1

Layer thicknesses of the individual Al and Ni layers of an AlNi multilayer metallization. The 400 nm thick AlNi metallization consists of three Al and three Ni layers with an Al layer at the interface to the substrate

Interdiffusion generally occurs when tempering multilayer metallizations the elements of the multilayer, where appropriate new phases are formed as well possibly interdiffusion and / or chemical reaction between one or both Elements of the multilayer and the substrate. In the specific case the AlNi multilayer is closed take into account that the heat treatment at temperatures above the melting temperature  rature of a multilayer component, namely the aluminum, must be carried out. Depending on the heating rate, the aluminum could melt in one case zen before an AlNi alloy can form or in another case by Interdiffu an AlNi phase with a higher melting point arise so quickly that the occurrence of Melt is prevented. The reaction between a molten metal and the substrate could also be different than the reaction between an AlNi alloy and the Substrate. To avoid the heat treatment producing unreproducible results leads, the RTA process (RTA = Rapid Thermal Annealing) into a diffusion temperature tion at temperatures below the melting temperature of the aluminum and a formation tempering at the high temperatures required to produce ohmic contacts divided up. The aim of the 60-minute diffusion annealing at 550 ° C is to achieve a homogeneous AlNi To generate metallization from the AlNi multilayer. The two-minute formation tempering then leads to the formation of ohmic contacts. The two-minute formation tempering is performed at 1200 ° C.

Fig. 3 shows the specific sheet resistances of AlNi contact metallizations as a function of the different mass fractions and the different heat treatments.

To measure the specific sheet resistance of the AlNi metallizations, the substrates were contacted at the top and bottom with two test needles each and the resistance along the metal path of length d and width W was determined by a four-point measurement. With a known layer thickness of 400 nm, the specific layer resistance ρ _{SH was then} sufficient

be calculated.  

In order to be able to compare the change in the specific sheet resistances with a variation in the aluminum content as a function of the substrate material (SiC or Si / SiO ₂ ) and the heat treatments carried out, all the measured values are shown in a diagram in FIG. 3.

When alloying a metal with foreign atoms, the mean free path length is reduced the electrons in the metal due to collisions between the electrons and the foreign atoms; What leads to an increase in sheet resistance. With completely miscible binary systems the specific sheet resistance reaches its maximum around the 50:50 Stoichiometry. In contrast, intermetallic phases can occur in a binary system the electrons due to the higher order state of the atoms in these phases move better in the alloy, the mean free path length increases and the specific one Sheet resistance correspondingly lower.

Due to the diffusion tempering at 550 ° C, the AlNi multilayers form the Ta belle 1 homogeneous alloys.

The specific sheet resistance reaches a maximum value of 7.10 ^-7 Ωm at an aluminum concentration of 20% and passes through a weakly defined minimum at an aluminum content of 60%, which may be caused by the fact that the diffusion tempering of the AlNi 60:40 metallizations has formed homogeneous Al ₃ Ni ₂ phase.

During the diffusion and forming tempering of the AlNi 0: 100 metallization on SiC, a strong reaction between nickel and SiC occurred, in which an Ni ₂ Si layer was formed. For this reason, the annealed AlNi 0: 100 metallization on SiC had an approximately three times greater sheet resistance than the same metallization on Si / SiO ₂ .

The layer wi changes for AlNi metallizations with 10% -40% aluminum content the additional two-minute tempering at 1200 ° C compared to the specimens annealed at 550 ° C for only one hour.

From 50% aluminum concentration of the metallization on Si / SiO ₂ or 60% aluminum content in the metallization on SiC, however, the specific sheet resistance increases by several orders of magnitude.

Fig. 4 shows a comparison of current-voltage characteristics of different AlNi- contact metallizations Diffusiontemperung after one hour at 550 ° C but without Formiertemperung.

After the diffusion tempering, current-voltage characteristics were measured on the substrates. It was found that all contacts, regardless of the AlNi stoichiometry, had non-ohmic behavior ( FIG. 4).

However, ohmic contacts can be formed according to the invention with a 2-minute forming tape tion between 800 ° C and 1200 ° C, preferably between 1000 ° C and 1200 ° C, completely particularly preferred at 1200 ° C from all metallizations with at least 10% aluminum produce content.

Fig. 5 illustrates very clearly the dependence of the specific contact resistance of the stoichiometry of the AlNi metallization. While the pure nickel contacts (AlNi 0: 100 metallization) show non-ohmic behavior even after the tempering of the formation, the alloying of 10% aluminum is sufficient to obtain ohmic contacts. On the other hand, if the metallization contains more than 50% aluminum, the specific contact resistance deteriorates by about two orders of magnitude, because the AlNi metallizations with 60% to 100% aluminum content melt during formation, do not form a coherent metal layer during solidification and therefore do not more the entire contact area is available for electricity transport.

The AlNi 50:50 metallization, with ρ _c = 1-2.10 ^-4 Ω.cm ^{2, has} the smallest value for the specific contact resistance of an aluminum-nickel contact metallization and is therefore suitable for contacting p-doped SiC substrates particularly good.

Fig. 6 shows an example of a cross section through a contact on a p-type SiC substrate. When contacting semiconductor components, the contacts usually consist of several layers. Fig. 6 illustrates that are possible with the inventive aluminum-nickel complex contact metallization contact structures.

The AlNi 50:50 contact layer, which is applied to the SiC substrate using one of the methods according to the invention, is additionally followed with barrier layers (100 nm chromium, 400 nm Mo, 50 nm chromium) after the diffusion and forming tempering of the AlNi contacts from a bond metallization of several chrome and platinum layers, a passivation from SiO ₂ and a protective metallization, which consists of a chrome-platinum multilayer, hen.

Claims (7)

1. Semiconductor substrate made of silicon carbide with at least one p-doped layer, with at least one SiO ₂ layer which has been grown on the p-doped SiC layer by deposition from the gas phase,
With a plurality of contact structures, which are introduced as contact openings into the SiO ₂ layer by means of standard photolithography using a mask set, wherein
the contact structures are formed by at least one layer of an aluminum-nickel layer with an aluminum content in the range from 40 at% to 60 at%, the aluminum-nickel layer is deposited in the contact openings and the contact structures to form an ohmic contact resistance between p -doped SiC layer and aluminum-nickel layer in the temperature range between 800 ° C to 1200 ° C are heat treated. 2. The semiconductor substrate according to claim 1, wherein the aluminum-nickel layer by sput onto an aluminum-nickel target from the gas phase onto the semiconductor substrate is divorced. 3. A semiconductor substrate according to claim 1, wherein the aluminum-nickel layer by diffusion sion stamping an aluminum-nickel multilayer is formed.   4. The production method for a semiconductor substrate as claimed in claim 1, in which a p-doped layer was epitaxially deposited on a SiC bulk material in a first step,
in a further step, an SiO ₂ layer is applied to the n-doped SiC layer by deposition from the gas phase at atmospheric pressure,
in a further step, contact structures are applied to the SiO ₂ layer by means of standard photolithography using a set of masks over a photoresist mask,
in a further step, the SiO ₂ layer is removed from the applied contact structures by reactive ion etching (RIE), so that contact openings are formed in the SiO ₂ layer,
in a further step the semiconductor substrate is cleaned and residues from the ion etching are removed,
in a further step the semiconductor substrate is coated with an aluminum-nickel layer in a vapor deposition system by electron beam evaporation of aluminum and nickel,
In a further step, the semiconductor substrate provided with the aluminum-nickel layer is subjected to a forming tempering in a radiation oven under an Ar protective gas atmosphere at temperatures between 800 ° C to 1200 ° C. 5. The method of claim 4, wherein the aluminum-nickel layer by electrons jet evaporation of an aluminum-nickel alloy is formed. 6. The method according to claim 4, wherein the aluminum-nickel layer by diffusion stamp an aluminum-nickel multilayer layer is formed and the diffusion stamping is carried out before the forming tempering.   7. The method of claim 6, wherein the diffusion annealing at 550 ° C for one hour is carried out.